Several mental disorders are closely associated with defects during neuronal development in early life. Abnormalities in neuronal differentiation and synaptogenesis may contribute to fragile X syndrome, Rett syndrome, autism, and other mental disorders. Post-transcriptional regulation by microRNAs (miRNAs) has emerged as an important mechanism for controlling gene expression in animal development; however, the exact functions of miRNAs in neuronal differentiation and function are poorly understood. Moreover, the genes and pathways regulated by miRNAs in the nervous system are largely unknown. miRNAs are 21-23-nt endogenous noncoding RNAs processed from 70-80-nt precursors that are mostly transcribed by RNA polymerase II and form stem-loop structures. miRNAs regulate gene expression by repressing translation or cleaving messenger RNAs. Estimated to comprise 1-5% of animal genes, miRNAs are thought to regulate the expression of a large number of target genes in developmental processes. A few miRNAs are specifically expressed in mammalian brains, suggesting unique regulatory roles in neuronal development and function. Indeed, miRNAs have been implicated in left/right neuronal asymmetry in Caenorhabditis elegans, photoreceptor formation and early neurogenesis in Drosophila, brain morphogenesis and neurogenesis in zebrafish, and neuronal differentiation in mammals. However, loss-of-function approaches have not been widely used to study the roles of miRNAs in the nervous system or other development processes. In Drosophila, for instance, only a few miRNAs have been studied in vivo using loss-of-function approaches, including miR-9a whose function in the specification of sensory organ precursors was reported by our laboratory. In this application, we propose to use in vivo genetic and molecular techniques and genomic approaches to elucidate the role of translational control by miRNAs in neuronal development in Drosophila. We will also investigate the underlying molecular mechanisms and validate and characterize one or two bona fide miRNA targets. The proposed studies will provide important novel insights into the neuronal functions of the miRNA pathway in intact animal models and will further our understanding of human mental disorders. Our findings may also help develop new avenues for therapeutic interventions for these devastating illnesses.